(CNN) -- The weekend accident that injured nine members of a circus troupe, along with two other people, during a Ringling Bros. and Barnum & Bailey performance in Providence, Rhode Island, was caused by a 5-inch D-ring that snapped, the lead investigator for the city's fire department said Monday.
"It was a single piece of equipment that failed," fire investigator Paul Doughty told reporters.
The steel caribiner was part of a rig designed to hoist performers by their hair into the air above the audience, where they would perform acrobatics. When it snapped, the rig fell to the ground to the confusion and horror of the audience.
What caused the D-ring -- also called a caribiner -- to snap remains under investigation, Doughty said.
The caribiner was rated for about 10,000 pounds, and the weight of the performers and the rig that fell was about 1,500 pounds, he said.
Doughty declined to say if the rigging appeared safe. But Providence Fire Chief Clarence Cunha said his crews would not rely on a single caribiner for safety.
First responders described a gruesome scene, with performers scattered on the floor, some suffering from compound fractures in which the bone protrudes through the skin. Many appeared to be in shock, they said.
"One of the girls looked up at me and very calmly but sadly said, 'I can't feel my legs,' " said Providence police Officer Sean Carroll, one of the first emergency officials to respond.
A performer on the ground and two other people were also hurt, officials said. Many suffered broken bones, officials said.
Of the nine people transported to the hospital, three remained critical on Monday, three were serious and the others were in good condition, Cunha said.
A circus spokesman called the accident "unprecedented."
"We have never had an accident like this with this number of performers injured," said Ringling Bros. spokesman Stephen Payne. "It really is a testament to, you know, their physical fitness and skills, that the injuries were not more severe than they were."
The injuries came after the failure of a rig holding eight members of the Medeiros Hair-Hang Act aloft by their hair. The rig collapsed as the audience looked on. Many suffered broken bones, Providence Public Safety Commissioner Steven Pare said Sunday. No spectators were hurt.
"I screamed. I'm like, that's not right," Chelie Barrie, a spectator, told WPRI. "You know, sometimes you're surprised and it's part of the show, but this clearly wasn't."
Rhode Island Hospital has confirmed that each of the eight women named by Ringling Bros. as part of the act are in the hospital: Viktoriya Medeiros, Widny Neves, Samantha Pitard, Viktorila Liakhova, Dayana Costa, Julissa Segrera, Stefany Neves and Svitlana Balanicheva.
The Medeiros Hair-Hang Act is touted on the Ringling Bros. website as a "one-of-a-kind act ... the brainchild of husband-and-wife team Andre and Viktoria Medeiros, (who) have devised and improved the mechanisms and methods making possible the myriad of maneuvers this troupe will perform for audiences."
"It is Andre's attention to every detail, even welding the three different rigs that the girls hang from, that keeps his troupe safe and sound each and every time the act is presented," according to the website.
The circus canceled both of its scheduled performances Monday amid the investigation, which Pare said is now being led by the federal Occupational Safety and Health Administration.
Payne said the circus is cooperating fully with investigators trying to understand what happened.
Elaine Alcorn, a circus performer who was not involved in the show, said "nothing is ever 100% sure" in a dangerous pursuit such as the circus.
"There can always be mistakes, there can always be failures in the rigging," she said.
It's something performers accept, she said.
"They don't call it death defying for nothing," she said. "As a circus performer it is our job to do the impossible, to stare death in the face and conquer it. We don't just do it for entertainment. We do it to inspire people, to inspire them to conquer their own fears or overcome their own obstacles."
CNN's Morgan Winsor, Kristina Sgueglia, Nick Valencia, Adrienne Zulueta, Dominique Dodley, Susanna Capelouto, Mark Morgenstein and Dana Ford contributed to this report.